1. Field of the Invention
The invention relates to asphalt production plants and, more particularly, relates to plants which are designed for the production of hot mix asphalt and which incorporate features for the reduction of VOC and NO.sub.x emissions.
2. Background of the Related Art
Plants for the production of hot mix asphalt (HMA) are well known and typically include a dryer for heating and drying stone aggregate; a coater-mixer for mixing the heated and dried stone aggregate with asphaltic cement (AC) in the form of liquid asphalt and possibly recycled asphalt product (RAP), dust or other materials; and a baghouse which removes dust from the gas stream(s) vented from the dryer and/or coater-mixer. The dryer typically comprises a heated parallel flow or counterflow inclined rotary drum dryer. The coater-mixer may comprise a pugmill or a fixed sleeve mixer of the type detailed below.
HMA plants typically present two major sources of emissions.
First, heat is typically supplied to the dryers of such plants via gas-fired burners, and emissions result through the incomplete combustion of fuel used to fire such burners and through combustion products from the burners. For instance, volatile organic compounds (VOCs) are produced through incomplete fuel combustion and oxides of nitrogen (NO.sub.x) are formed as combustion products. Release of these emissions has historically been controlled through burner design. However, adequately controlling emissions through burner design alone is difficult, principally because a tradeoff is often required between the reduction of VOCs and NO.sub.x. That is, VOC emissions are minimized at or near stoichiometric combustion conditions. NO.sub.x emissions, on the other hand, are minimized through off-stoichiometric combustion. Control of burners to minimize VOCs thus tends to increase NO.sub.x levels, and vice versa.
Faced with these difficulties and with the imposition of increasingly strict emission regulations, designers are turning to alternative measures such as flue gas recirculation to control emissions, particularly in certain areas of the country such as Southern California where emission regulations are particularly strict. These other measures have limited effectiveness and will likely prove inadequate in the long run as emission regulations become increasingly strict. The need has therefore arisen to provide better control of emissions from the dryers of HMA plants.
The second major source of emissions from HMA plants is from the coater-mixers in which the mixing of AC with hot aggregate tends to release VOCs through volatilization of VOCs in the AC. VOC production is particularly problematic in applications in which the AC includes RAP since the mixing of the RAP with the heated virgin aggregate produces relatively high amounts of hydrocarbon-laden vapors, collectively known as blue smoke. The amount of blue smoke formed during operation of coater-mixers can be reduced by limiting the exposure of the AC to the HMA to the minimum required for adequate mixing and coating, thus reducing the amount of volatilization; by reacting those VOCs which are formed with water vapor driven from the hot aggregate; and by promoting combustion of released hydrocarbons. The latter elimination technique may involve drawing the blue smoke into a dryer and recirculating the blue smoke into the burner flame, or directing the blue smoke from the coater-mixer to the air inlet port of the burner for the dryer. Both combustion techniques require that the blue smoke be burned, and such burning, even if effective, risks potentially adverse consequences such as partial burner flame quenching. The need has therefore arisen to reduce the emission of blue smoke from the coater-mixers of HMA plants.
It has long been known in other industries to reduce the levels of NO.sub.x and, to a lesser extent, VOCs in flue gases by reacting the gases with a metal catalyst provided on a ceramic substrate. This technique has, however, never been accepted in the HMA industry possibly because (1) the gas stream exhausted from the dryers of such plants are not at a sufficiently high temperature to react with any known catalyst, and (2) the dust in the gas streams tends to poison or to quickly render ineffective most catalysts.